This method and apparatus are particularly applicable to measuring variation in the elongation of an optical fiber cable core including grooves for housing optical fibers and subjected to tension which causes the core to lengthen elastically prior to having optical fibers inserted in its grooves. Thereafter the tension is released in order to obtain excess length of fiber in the grooves, thereby preventing the fibers from being subjected to elongation as severe as that to which the cable may be subjected after manufacture during handling operations and during laying. Grooved cores of this kind are described in published French patent specifications Nos. 2 312 788 and 2 544 874, with patent specification No. 2 544 874 being in the name of the Assignee. The invention also applies to measuring the elongation of wires or cables during manufacture.
When manufacturing optical fiber cables, and in particular submarine cables, it is necessary to allow for considerable traction forces which will be exerted on the cable while it is being laid and possibly also while it is being raised in order to perform a repair thereon. This is because optical fibers are capable of withstanding very little elongation without breaking (about 1% to 1.5%), whereas the plastic and metal items constituting the cable core and its armoring are likely to elongate much more during traction. It is thus necessary to provide excess optical fiber length within the cable so that the fibers themselves are put under traction as late as possible when the cable as a whole is elongated, as described in German patent specification No. 2 519 050.
In current versions of this type of cable, optical fibers are individually disposed in recesses which are formed (often helically) in the periphery of a core of plastic material provided with an axial metal cable reinforcing of strength member made of a steel having a high elastic limit. The recesses are wide enough and deep enough to allow the fibers to slide freely and even to take up an undulating path which is considerably longer than the length of the core.
This excess optical fiber length is obtained very simply by placing the fibers in their recesses in a zone of the manufacturing line where the grooved core is highly elongated under the effect of a traction force of several tens of kilograms. This force is then released and the grooved core returns to its initial length so that the length of fiber disposed in the recesses is greater than the length of the cable. As a result it is frequently possible to elongate such a cable by as much as 1.4% before the fibers themselves are subjected to a traction force.
It will readily be understood that it is most important to be able to accurately control said excess length during cable manufacture, since if too little excess length is provided then optical fibers may break when the cable is under traction, while if too much excess length is provided then both the cost and the attenuation of the cables suffer, in particular because of the manufacturing which is likely to occur in the fibers. Further, the measurement of core elongation must be performed as much as possible while the cable is moving during manufacture so as to be able to correct any defect that may occur before it is too late. Since it is very difficult to measure the length of the fibers themselves, measurements are performed on variations in the elongation of the central carrier including the grooved core between zones where it is subjected to little tension and zones where it is put under considerable tension in order to receive the optical fibers.
The present invention makes use of a technique whereby the very long element includes a magnetizable material which is marked by magnetizing said material. French Pat. No. 2 466 746 has already proposed using magnetic marking for measuring the length of running objects, such as tubes or metal cables, but this method is incapable of showing up small variations in length when measuring variations in elongation between a zone where a core is put under low tension and a zone where it is put under much greater tension, followed by a zone where the tension is released, and in particular when the very long element is subjected to non-uniform elongations as it moves.
British patent specification No. 2 131 553 proposes detecting unwanted elongation of a conveyor belt by comparing the distance between two magnetic marks applied to the belt with the fixed reference distance that ought to exist between said marks on the belt. However, this method only gives approximate results and is only applicable to a band or other analogous item running round a closed loop.
The aim of the present invention is to provide a method and an apparatus enabling variation in the elongation of a very long element to be measured with high accuracy and at a high frequency, when such variations in elongation are relatively small and non-uniform, and when the element in question is running along an open circuit in a manufacturing line.